Finnish Patent Application No. 912498 discloses a multi-layer sleeve connection, whose layers are made by extruding, injection molding, or by a crosslinking process. One layer is a reinforcement layer which may also contain metal. The sleeve further comprises resistance wire winding mounted on its inner surface for welding the sleeve to the pipe connected to it.
Application EP 055 891 discloses the technology that is most commonly used at present. This technology comprises manufacturing at first a preform by injection molding, winding a resistance wire on the preform, and injection molding a reinforcement layer on top of the preform and the resistance wire, whereby the preform and the reinforcement portion are at least partly welded together.
Application GB 2 232 330 discloses a sleeve manufacturing method comprising winding a thermoplastic tape which is obtained from an extruder and which is in a molten or soft state on a mandrel, and fusing the edges of the windings together.
Prior art solutions are also disclosed in DK 162588 and WO 9515253.
One of the drawbacks of the prior art solutions mentioned above is that it is uneconomic to manufacture particularly large branch fittings with the above-mentioned conventional methods, since a large amount of material to be injected requires a long cooling time. In addition, the structures are often massive in order for the required strength to be provided, and the tool costs as well as investments in the injection molding machines are extremely high.